1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a heat fixing apparatus to be mounted on an image forming apparatus such as a copying machine or a printer utilizing electrophotographic recording technology.
2. Description of the Related Art
In a prior image forming apparatus, an image is formed on an image bearing member by an appropriate image forming process such as an electrophotographic process, then such image is either directly or indirectly borne on a recording material, and such unfixed image (toner image) is heat fixed, by a fixing apparatus, on the surface of the recording material as a permanently fixed image. For such fixing apparatus, an apparatus of heat roller type has been widely utilized. Also in recent years, an apparatus of film heating type has been commercialized from the standpoints of quick starting and energy saving. Also proposed is an apparatus of electromagnetic induction heating type, in which heat is generated in a film itself constituted of a metal.
In the following, a fixing apparatus of film heating type will be described as a representative example.
Such a film heating type of a fixing apparatus is described, for example, in Japanese Patent Application Laid-Open No. S63-313182, Japanese Patent Application Laid-Open No. H02-157878, Japanese Patent Application Laid-Open No. H04-44075, and Japanese Patent Application Laid-Open No. H04-204980.
FIG. 9 illustrates schematic structure of an example of the fixing apparatus of film heating type.
The fixing apparatus 1 of this example includes a fixing film 10 of a cylindrical shape, formed by a heat-resistant resin such as polyimide and constituting a rotary member unit 2 at a heat source side. The cylindrical film 10 is loosely fitted around a film guide member 16 having a substantially semicircular trough-shaped cross section.
Inside the film guide member 16, there is provided a laterally oblong linear heat-generating unit (ceramic heater) 15 of a low heat capacity, which is prepared by forming a heating member 13 on a heater substrate 14 constituted of a ceramic substrate such as of alumina (Al2O3). Glass or a fluorinated resin is coated as an overcoat layer (protective layer) 12 so as to cover the heating member 13.
An elastic pressure roller constituting a pressurizing rotary member 3 is pressed to the ceramic heater 15 across the fixing film 10, thereby forming a fixing nip portion N.
The pressure roller 3 is rotated by a drive unit M, counterclockwise as indicated by an arrow. The rotating drive of the pressure roller 3 applies a rotary power to the fixing film 10, by a frictional force between the pressure roller 3 and the external surface of the fixing film 10. Therefore, the fixing film 10 rotates clockwise, as indicated by an arrow, with the internal surface thereof in sliding contact with the ceramic heater at the fixing nip portion N, with a peripheral speed approximately corresponding to the peripheral speed of the pressure roller 3. Thus, the fixing apparatus of this example is a fixing apparatus of so-called film heating type, and a pressure roller drive type.
The film guide member 16 serves to support the heat-generating unit 15, to support the fixing film 10 and to stabilize the movement of the film 10 in rotation.
The fixing film 10 is constituted of an unillustrated polyimide base layer (about 50 μm), an electroconductive primer layer (5 μm), and a release layer of a fluorinated resin provided thereon. Also in a fixing apparatus for a full-color image forming apparatus, designed for fixing a color image having a larger toner amount, there is employed, in order to improve the fixing ability, a fixing film having a silicone rubber layer (100 μm or larger) between the conductive primer layer and the release layer of fluorinated resin. This allows the fixing film 10 to follow the surface irregularities of the recording material, thereby avoiding image defects such as unevenness in gloss.
The temperature of the fixing nip portion N is regulated by a temperature control system including a temperature detection unit 11, such as a thermistor, mounted on a rear surface of the ceramic heater 15.
A recording material P, conveyed from an unillustrated image forming portion and bearing an unfixed toner image T, is introduced between the fixing film 10 and the pressure roller 3 in the fixing nip portion N, with the toner image bearing surface upwards, namely opposed to the surface of the fixing film. In the fixing apparatus 1, the image bearing surface is pinched and conveyed in the fixing nip portion N, in close contact with the external surface of the fixing film 10 and together with the fixing film 10, through the fixing nip portion N. While the recording material P is pinched and conveyed, together with the fixing film 10, through the fixing nip portion N, the unfixed toner image T on the recording material P is heat fixed thereto. The recording material P, upon passing through the fixing nip portion N, is separated from the external surface of the rotary fixing film 10 and is further conveyed for discharging.
In the foregoing, a fixing apparatus of film heating type has been described as an example, but, in any fixing apparatus of different heat-generating system, the unfixed toner image on the recording material is heat fixed thereto by the heat from a heat source-side rotary member unit.
The heat (energy) from the heat source-side rotary member unit is transmitted not only to the unfixed toner image and the recording material, but also to the exterior through the air around the heat source-side rotary member unit and through other members in contact with the heat source-side rotary member unit. It is preferable that energy not contributing to the fixation is made as little as possible. Therefore, following technology is proposed as a heat shield method for preventing the heat, generated from the heat source-side rotary member unit, from diffusion to the exterior of the fixing apparatus.
FIG. 10 illustrates an example of the heat shield technology for the fixing apparatus of heat roller type. In this fixing apparatus, a heat shield cover 200 is provided around the fixing roller 4, which is one of the members constituting the heat source-side rotary member unit 2. Such construction intercepts the heat radiation from the fixing roller 4 and also suppresses the heat transmission by a convection in the air layer around the fixing roller 4, thereby preventing the heat diffusion to the exterior of the fixing apparatus. In addition, a heat shield effect of an air layer in a space S, surrounded by the heat shield cover 200 and an outside frame 5 of the heat source-side rotary member unit 2, prevents an unnecessary heat transmission to the exterior of the fixing apparatus.
Similar heat shield technologies are disclosed in Japanese Patent Application Laid-Open No. H05-134572 and Japanese Patent Application Laid-Open No. H05-188805. In either technology, a heat shield cover 200 or a reflecting plate is disposed outside the heat source-side rotary member unit 2 to intercept the externally diffusing heat, thereby improving the thermal efficiency of the entire fixing apparatus. However, the fixing apparatus of the aforementioned construction is associated with following shortcomings.
For example, as illustrated in FIG. 10, the heat shield cover 200 is in contact with the external frame 5 of the heat source-side rotary member unit 2 (namely frame of the fixing apparatus).
Such contact of the heat shield cover 200 and the frame 5 causes a heat transmission from the heat shield cover 200, heated by the heat radiation from the fixing roller 4, to the external frame 5 of the heat source-side rotary member unit 2, thereby heating the frame 5. The frame 5 is positioned closer to the external wall of the printer, in which the fixing apparatus is mounted, and is easier to cause heat dissipation. Such construction inducing an easy heat transmission to the frame 5 results in a waste of energy. Thus, the technologies of the prior references still have room for improvement.